This invention relates to filtering membranes and particularly to modules of immersed, suction driven, filtering membranes used to filter water or wastewater and cleaned in part by scouring air bubbles.
Submerged membranes are used to treat liquids containing solids to produce filtered liquid lean in solids and an unfiltered retentate rich in solids. For example, submerged membranes are used to withdraw substantially clean water from wastewater and to withdraw potable water from a lake or reservoir.
Immersed membranes are generally arranged in elements or modules which comprise the membranes and headers attached to the membranes. The modules are immersed in a tank of water containing solids. A transmembrane pressure (xe2x80x9cTMPxe2x80x9d) is applied across the membrane walls which causes filtered water to permeate through the membrane walls. Solids are rejected by the membranes and remain in the tank of water to be biologically or chemically treated or drained from the tank.
U.S. Pat. No. 5,639,373 issued to Zenon Environmental Inc. on Jun. 17, 1997 describes one such module using hollow fibre membranes. In this module, hollow fibre membranes are held in fluid communication with a pair of vertically spaced headers. TMP is provided by suction on the lumens of the fibres through the headers. Similar modules are shown in U.S. Pat. No. 5,783,083 issued to Zenon Environmental Inc. on Jul. 21, 1998, PCT Publication No. WO 98/28066 filed on Dec. 18, 1997 by Memtec America Corporation and European Patent Application No. EP 0 931 582 filed Aug. 22, 1997 by Mitsubishi Rayon Co., Ltd.
To clean such membrane modules, bubbles are introduced to the tank through aerators mounted below or near the bottom of the membrane. The bubbles rise to the surface of the tank water and create an air lift which recirculates tank water around the membrane module. The rising bubbles and tank water scour and agitate the membranes to inhibit solids in the tank water from fouling the pores of the membranes. Further, there is also an oxygen transfer from the bubbles to the tank water which, in wastewater applications, provides oxygen for microorganism growth.
One concern with such aerated immersed membrane modules is that the tank water to move in a generally steady state recirculation pattern in the tank. The recirculation pattern typically includes xe2x80x9cdead zonesxe2x80x9d where tank water is not reached by the recirculating tank water and bubbles. The parts of the membranes in these dead zones are not effectively cleaned and may be operating in water having a higher concentration of solids than in the tank water generally. Accordingly, the affected parts of these membranes quickly foul with solids. This problem persists even in modules where membranes are installed with a small degree of slack to allow the membranes to move and shake off or avoid trapping solids. The movement of water in the tank encourages the slackened membranes to assume a near steady state position near the headers which interferes with the useful movement of the membranes. As a result, the entire surface of the membranes is not effectively cleaned and parts of the membrane foul rapidly. In wastewater applications in particular, sludge often builds up around the membranes in an area directly above the lower header and an area directly below the upper header. 15% or more of the surface area of the membranes may quickly become covered in sludge and lose nearly all of its permeability.
U.S. Provisional Application No. 60/103,665, filed Oct. 9, 1998 by Cote, made non-provisional as U.S. application Ser. No. 09/414,370 filed Oct. 7, 1999 by Cote, describes a cassette of filtering membranes suspended from a horizontal pivot located above the center of gravity of the cassette. Aerators are provided below the cassette on opposite sides of the pivot. A supply of air is cycled from the aerators on one side of the pivot to the aerators on the other side. This causes the cassette to oscillate, the aerated side rising while the non-aerated side falls.
PCT Application No. PCT/CA99/00940, filed on Oct. 9, 1999 by Zenon Environmental Inc. et al, and continued in the US by Cote et al. on Jan. 19, 2000 as application Ser. No. 09/488,359, describes among other things a method and apparatus for reducing the build up of sludge on vertical membranes near the headers. The apparatus includes an aeration system having a plurality of distinct branches and one or more aerators in fluid communication with the each distinct branch. An air supply provides an initial air flow at an initial flow rate and a valve set is provided in fluid communication with the air supply and having distinct outlets in fluid communication with the distinct branches of the air distribution system. The valve set is operable to (i) split the initial air flow such that at any point in time at least one of the distinct branches of air distribution system receives air at a higher flow rate and at least one other of the distinct branches of the air distribution network receives air at a lower flow rate, the lower flow rate being less than one half of the higher flow rate, and (ii) switch which branch or branches of the air delivery network receive air at the higher flow rate and the lower flow rate in repeated cycles of very short duration. The aerators associated with a first distinct branch of the air delivery system are interspersed with the aerators associated with a second distinct branch of the air delivery system. With sufficiently short cycle times, the water to be filtered moves horizontally under transient flow. When used with membranes oriented vertically between upper and lower headers, the horizontal and transient movement encourages movement of the fibres and penetration of the tank water into the fibres to help prevent a build up of sludge around the headers.
It is an object of the present invention to provide an element or cassette of immersed, suction driven, filtering membranes used to filter water or wastewater which may be cleaned in part by scouring air bubbles. The present invention is particularly adapted to filtering water with a high concentration of suspended solids.
In some aspects, the invention is directed at an element of filtering hollow fibre membranes having an upper header and a lower header. A plurality of hollow fibre membranes are attached to and suspended between the headers for collecting permeate through at least one of the headers. The lower header is movable between a first position in which the fibres are substantially elongated and second position in which the two headers are closer to each other by between 0.1 and 5% of the un-potted length of the membranes. The weight of the lower header is sufficient to keep the lower header in the first position in substantially quiescent water while allowing the lower header to rise to the second position in upwardly flowing air and water.
A module of filtering hollow fibre membranes is made by attaching one or more of the elements to a frame. The frame fixedly secures the upper header but merely restrains the lower header while allowing it to move between the first position and the second position. Aerators are mounted generally below the elements and supply scouring bubbles to each element at a higher rate and then at a rate less than one half of the higher rate in repeated cycles. The cycles are preferably between 10 seconds and 60 seconds in duration. The lower headers rise to the second position when bubbles are supplied at the higher rate and fall to the first position when bubbles are supplied at the lower rate.
In other aspects, the invention is directed at a process for treating water with filtering hollow fibre membranes of the type that have a plurality of hollow fibre membranes attached to and suspended between a pair of headers. The membranes are provided in elements, each element being a rectangular skein of hollow fibres having an effective thickness of between 4 and 8 sheets of hollow fibres. Adjacent elements are horizontally spaced apart, preferably by at least one third of the width of the headers measured in the direction of the horizontal spacing. Suction is applied to the interior of the hollow fibre membranes to withdraw a filtered permeate. During permeation, scouring bubbles are provided from below the elements. The supply of the scouring bubbles varies between a higher rate and a rate less than one half of the higher rate in repeated cycles. The cycles are between 10 seconds and 60 seconds in duration. The distance between the headers is between 95% and 99.9% of the un-potted length of the hollow fibre membranes for at least a substantial part of the time during which bubbles are supplied at the higher rate.
In yet other aspects of the invention, at least one header moves during permeation between the first position and the second position while scouring bubbles are supplied in repeated cycles as described above. Preferably the lower header moves upwards to the second position when bubbles are supplied at the higher rate and moves downwards to the lower position when bubbles are supplied at the lower rate.